1. Field of the Invention
The present invention relates to a bumper for relaxing and absorbing an impact produced by driving a piston mainly in a driving tool such as a pneumatic tool or a gas combustion type driving tool, and also relates to the driving tool.
2. Related Art
For example, a driving tool which drives a piston by compressed air thereby to drive a fastener such as a nail, a drive screw, a staple, or the like by a driver connected to the piston and drive out the fastener toward a member to be driven is provided with a cushioning mechanism for absorbing an impact of a piston. This cushioning mechanism is constituted by a cylindrical bumper which is usually disposed below a cylinder, receives a lower surface of the piston, and absorbs the impact of the piston.
As such the bumper, Patent Document 1 discloses a bumper in which an inner diameter and an outer diameter of a lower portion is made larger than an inner diameter and an outer diameter of an upper portion, a large space is formed on an inside of the bumper lower portion, and a clearance between a driver and a driver guide hole is made small. In this bumper, the clearance is closed when the impact of the piston is applied, thereby to compress the air trapped in the lower space, and an absorption effect of the impact is enhanced by means of synergy between the elasticity of the bumper and air cushion.
Patent Document 2 discloses a hollow-cylindrical bumper which has an external shape that an outer diameter of a bumper upper portion is small and an outer diameter of a bumper lower portion is large. This bumper has an inner diameter of a hollow part in which a lower inner diameter is larger than an upper inner diameter, so as to form an air-gap portion. From this air-gap portion, a deformation of the compressed bumper can escape and the deformation in the compressed direction is promoted, whereby an impact-absorption advantage of a piston is enhanced.
Patent Document 3 discloses a vertically-long and cylindrical bumper. In this bumper, an upper portion is thick and its outer diameter is made substantially the same as the inner diameter of a housing. An intermediate portion bulges so as to follow a bulging-out inner peripheral surface of a lower portion of the housing. A bumper lower portion is formed thin and provided with an air gap. Thereby, the bumper lower portion is easy to deform, and this deformed portion is escaped into the air gap thereby to enhance an impact absorption advantage of a piston.    [Patent Document 1] JP-A-08-336776    [Patent Document 2] JP-A-07-241783    [Patent Document 3] JP-U-07-017481
The bumpers described in the above-mentioned Patent Documents 1 to 3 are so designed as to receive the lower surface of the piston driven by high air pressure or high combustion pressure at an upper chamber of a cylinder, and to absorb the impact at the bumper upper portion or the bumper lower portion. In any of the bumpers, the upper portion is asymmetric in shape to the lower portion, and the deformation by flexure produced upon reception of the impact tends to concentrate on the upper portion or the lower portion. Since such the structure instantaneously absorbs the impact and stress concentrates on only the deformed portion, only the deformed portion deteriorates. Namely, since the flexure at the upper portion and the flexure at the lower portion are not uniform, resultantly, a durability of the bumper locally lowers.
Further, at the lower portion of the cylinder, an exhaust port communicating with a blowback chamber is formed. When the piston is driven, the air compressed in a lower chamber of the cylinder is stored through the exhaust port in the blowback chamber. By feeding back the air in this blowback chamber from the exhaust port to the lower chamber of the cylinder, the piston which has descended up to a bottom dead center ascends up to a top dead center. Since the exhaust port is arranged at the cylinder portion corresponding to the outer portion of the bumper upper portion, every time the bumper is compressed upon reception of the impact by the piston and bulges outward, the bulging-out portion comes into strong contact with an opening end of the exhaust port. Therefore, during the repeat of the contact, the surface of the bumper is damaged, and the durability is impaired.